This invention relates to pumps for delivering fluids to a patient. More specifically, the invention relates to a system and method of varying the flow rate of fluid to a patient through a tube based on monitored characteristics of the tubing and fluid within the tubing.
There exists no highly integrated hybrid sensor system capable of implementing three sensing functions in a single system. While non-invasive flow meters that use ultrasonic sensors or lasers exist along with non-invasive thickness gauges that use ultrasonic sensor capability along with separate sensors that can detect air bubbles and fluid within tubing, no sensors exist that can monitor multiple characteristics of tubing and fluid within the tubing in order to alter the flow rate of fluid from the pump through the tubing.
Ultrasonic sensors have been utilized in many industries and in particular, the medical industry for many years. However, ultrasonic sensors are high cost and have impedance matching problems and thus this basic electric piezoelectric technology needs to be replaced or enhanced.
Specifically, there are two impedance matching problems that exist with current piezoelectric ultrasonic transducers. First, there is a problem with acoustic impedance matching. Specifically, a quite low percentage of ultrasonic energy generated from a piezoelectric transducer can be delivered to the target (a biological tissue, a polymer tube, etc.). A very large percentage of ultrasonic energy that is generated from the piezoelectric transducer is bounced back and forth in the piezoelectric film instead of being sent out for detection or imaging purposes.
The second problem associated with piezoelectric transducers is electric impedance matching. Specifically, piezoelectric material, due to its material property of having high electrical resistivity in combination with a very thin film that is used, has a poor electrical impedance matching to regular electrical conductors. Thus, special electrical impedance matching layers have to be added. This increases manufacturing costs. The acoustic impedance of one particular conventional piezoelectric transducer (PZT) disk, water and air is 30 Mrayl (106 KG/M2S), 1.5 Mrayl, and 400 rayl (KG/M2S), respectively. With such a significant amount of impedance mismatch a large percentage of the ultrasonic energy bounces back at the transducer/fluid interface that the acoustic band width of the transducer is relatively narrow. Although special designs like having added impedance matching layers on the transducer surface have been utilized to alleviate the problem, the poor coupling efficiency from the transducer to the target is still a fundamental limitation of piezoelectric ultrasonic systems used in medical applications.
Peristaltic pumping has been utilized in many industries and in particular, the medical industry from many years. As such, there exist many improvements in the art. Below are a few techniques that have been used in the past for compensating for tubing wear.
The most common technique for compensating for tubing wear is to include an algorithm in the pump that adjusts the speed of the pumping mechanism based on the amount of time the pump is running. Such an algorithm is developed based on running flow rate accuracy tests for extended periods of time. The pump is run at a single set mechanism speed and data is collected over time to show how the flow rate is affected as a function of time. This process is repeated over several set mechanism speeds to provide a full characterization of how flow rate is affected by tubing wear. Once this data is obtained, an algorithm can be developed. Typically, for a given mechanism speed, flow rate begins to diminish over time as the tubing wears. In order to compensate for this affect, the algorithm would adjust the mechanism speed (e.g. increase speed) such that the steady-state flow rate could be maintained over time.
Several algorithms are known in the art. Nose et al., U.S. Pat. No. 7,284,956 describes in general one such invention using any number of feedback controllers, a mechanism, and sensors to maintain a pump flow rate at its operating set point. Another algorithm, specific to peristaltic pumping that is well known in the art is to utilize the internal pressure profiles which exist in the fluid inside the tubing.
Another technique for compensating for tubing wear is to implement sensors that directly or indirectly characterize the fluid flow and put the data from the sensors into a control system for closed-loop feedback. Among the most common sensors that have been proposed for this are the indirect sensors. These sensors measure a characteristic that is indicative of or can be correlated to the flow rate. Some common methods are: 1) measurement of the tubing dimensions (inside diameter, outside diameter, wall thickness, etc.); 2) measurement of the force to occlude/pump the tubing; and 3) measurement of the pressure inside/outside the tubing.
These techniques all rely on measuring properties that impact flow rate. Therefore, monitoring how these properties change over time also indicates how the flow rate is changing over time due to tubing wear. Another more rational approach is to utilize a sensor that directly indicates flow rate, a flow sensor. Many types of flow sensors exist (optical, ultrasonic, magnetic, etc). While this is solid solution, it is quite disadvantageous for the medical device market as it comes at a particularly high cost of implementation and has clinical issues.
Another way this problem has been overcome is to improve the tubing material itself. That is, to design and manufacture a tubing material that has reduce wear characteristics and can withstand the many cycles of compression that is undergone in peristaltic pumping. As such, materials such as silicone and Tygon™ are among the most popular for peristaltic tubing applications. As opposed to thermoplastic materials, such as polyvinylchloride (PVC), they are highly resilient and compliant and have been shown to perform well in long term pump applications. Some disadvantages to this solution are the higher cost for this material, the difficulty in joining or bonding the material to other polymer components, and although the wear is greatly improved, the tubing still does degrade over time and is not a complete solution to the problem.
Thus, a principal object of the present invention is to characterize tubing, fluid and fluid flow properties during pumping of fluid with a medical pump.
Yet another object of the present invention is to be able to have a lower device/system cost with added functionality and better accuracy.
These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.